The kind of polypropylene in general use, for example, for packaging and container functionality, is isotactic polypropylene. It is typically described as having the methyl groups attached to the tertiary carbon atoms of successive monomeric units on the same side of a hypothetical plane through the main chain of the polymer. Isotactic polypropylene lacks clarity and thus is not useful in cases where this is important.
Another kind of polypropylene is syndiotactic polypropylene. It may be described as having the methyl groups attached to the tertiary carbon of successive monomeric units on alternate sides of a hypothetical plane through the main chain of the polymer. Syndiotactic polypropylene is clear, that is it does not have the lack of clarity characteristic of isotactic polypropylene.
There are two types of syndiotactic polypropylene. One of these types is referred to as being made by a chain-end control mechanism and contains defects of the rmr type. NMR analysis for this kind of structure is shown in Zambelli, et al., Macromolecules, 13, 267–270 (1980). The most syndiospecific polypropylene of this type made before this invention has [rrrr] pentad content of 0.63 (described in Pellecchia, C., et al., Macromol. Rapid Commun. 17, 333–338 (1996)) which limits the usage since the lower the [rrrr] pentad content, the lower the melting point. For example, a container made from polypropylene with [rrrr] pentad content of 0.63 will melt on contact with boiling water and thus is unuseful for containers for hot liquids. We turn now to the other type of syndiotactic polypropylene. It is referred to as being made by a site-control mechanism and as containing defects of the rmmr type. This type of polypropylene is described in European Patent Application Publication 0351391 A2 (published Jan. 17, 1990). Highly syndiotactic polypropylene ([rrrr]=0.97) has been made by a site-control mechanism See Ewen, J. A., et al., J. Am. Chem. Soc., 110, 6255–6256 (1988), Herzog, T. A., et al., J. Am. Chem. Soc. 118, 11988–11989 (1996), and Veghini D., et al., J. Am. Chem. Soc. 121, 564–573 (1999). This kind of syndiotactic polypropylene has not yet been commercialized apparently because of processing and/or economic factors.